Electrical “noise” is a problem for analog and digital circuit designers. This is particularly true for mixed signal circuits that include both digital and analog sub-circuits. FIG. 1 illustrates an exemplary conventional system 10 having both digital and analog blocks operating simultaneously. This system 10 includes a digital logic block 12 (noise generating device), an analog block 14 (noise sensitive device), a power supply 16, and a ground 18. An output from the analog block 14 is an output from the system 10.
Noise or voltage spikes are illustrated in FIG. 1 as a wavy power line and may be caused by, for example, variations or ripple in power supply voltage lines, by ground bounce, and by false triggering of signal transitions. Digital circuits generate noise when they switch between high and low levels. As illustrated in FIG. 1, switching in the digital block 12 causes a noise ripple on the power supply line 16. These noise ripples are input to the analog block 14, which passes them through to the output of the system 10. In some instances, the analog block 14 may actually amplify the input noise. Noise decreases the overall signal to noise ratio (SNR) for the system 10, which reduces the quality of the output.
Although it is possible to minimize or eliminate noise that is generated periodically and in a predetermined manner, by using various techniques, noise generated by switching digital circuits typically does not follow any set pattern and is not known what noise will be generated before it is generated, and therefore cannot be predicted, nor cancelled.
Digital circuit blocks tend to be less susceptible to being affected by noise than analog circuit blocks because digital blocks typically have larger signal margins than do analog blocks. Analog blocks tend to be quite sensitive to noise, and power-supply ripple may be directly visible on analog outputs, causing lower SNR and relatively poor performance. Thus, it is important to reduce or eliminate noise in circuits where possible.
One known solution to minimize noise is to provide separate power supplies and ground circuits to digital and analog circuits. Having separate supplies and grounds tend to improve the noise problems, but generally do not eliminate them. Other solutions include using grounding and shielding techniques, to varying degrees of success. Another commonly used technique is to slow down the edges of transitioning signals, thereby reducing the rate of change of voltage (dV/dT), and reducing the noise generated by the transitions. These techniques may be helpful but often are insufficient, especially at high frequency.
Embodiments of the invention address these and other limitations in the prior art.